The afterglow and elliptical host galaxy of the short γ-ray burst GRB 050724

Despite a rich phenomenology, γ-ray bursts (GRBs) are divided ¹ into two classes based on their duration and spectral hardness - the long-soft and the short-hard bursts. The discovery of afterglow emission from long GRBs was a watershed event, pinpointing² their origin to star-forming galaxies, and hence the death of massive stars, and indicating ³ an energy release of about 10⁵¹ erg. While theoretical arguments⁴ suggest that short GRBs are produced in the coalescence of binary compact objects (neutron stars or black holes), the progenitors, energetics and environments of these events remain elusive despite recent ^5-8 localizations. Here we report the discovery of the first radio afterglow from the short burst GRB 050724, which unambiguously associates it with an elliptical galaxy at a redshift⁹ z = 0.257. We show that the burst is powered by the same relativistic fireball mechanism as long GRBs, with the ejecta possibly collimated in jets, but that the total energy release is 10-1,000 times smaller. More importantly, the nature of the host galaxy demonstrates that short GRBs arise from an old (>1 Gyr) stellar population, strengthening earlier suggestions^5,6 and providing support for coalescing compact object binaries as the progenitors.